First Ringneck Passed

Yesterday I cleaned out the Pheasant pen down to the soil. Then I dusted the soil with DE, then put in a layer of animal bedding, then put in an inch or so of pine shavings. As a treat, I gave my ringnecks some sunflower seeds, spinich and broc. This morning when I went to give them their daily fresh water, my roosters was dead. He was cold and sort of stiff so he must have died early last night. He was only four months old. My question is what could have caused his death? He was healthy as far as I could see. He ate well last night. Could the stress of cleaning the pen or maybe bacteria in the air have killed him? I completely clean all of my pens every three months. This was the first time that he experienced it though. The hen that was in with him is fine...???